Private base stations

ABSTRACT

A private base station ( 231 ) provides communication directly between a cellular mobile telephone ( 201 ) operable within a cellular network and a public switched terrestrial network ( 116 ). The base station is configurable to establish communication to a mobile telephone while minimizing interference with similar base stations. The base stations may transmit on similar frequencies but within different time division multiplexed time slots.

FIELD OF INVENTION

[0001] The present invention relates to private base stations arrangedto provide communication directly between a cellular mobile telephone,operable within in a macro cellular network, and a public switchedterrestrial network.

BACKGROUND

[0002] In cellular networks, it is necessary to adopt an optimizedcoverage policy in terms of providing sufficient signal strengths atlocations within the region covered while minimizing infrastructureexpenditure. Experience has shown that user penetration is pricesensitive and customers will tolerate a degree of signal degradationwhen the level of service is reflected in their connection charges.Thus, it is theoretically possible to provide superb radio coverage overwide geographical areas, however the cost of doing this often provesprohibitive and would result in a reduced customer penetration withsystem operators being unable to establish a profitable network.

[0003] In areas where it is not possible to obtain a sufficiently strongcommunication signal, other modes of communication, such as connectionto a public switched telephone network are often available. Furthermore,customers may be reluctant to use mobile equipment if less expensiveland based systems are readily accessible at a particular location. Thiswill tend to discourage customers from regularly using mobile equipmentwhich in turn may reduce overall system use due to the mobile equipmentnot being readily accessible.

[0004] A private base station provides a mechanism for relieving theabove problems in that, in preference to being connected to anestablished cellular network, a mobile phone user may establish aconnection to a dedicated private base station which in turn allows themobile telephone equipment to be used to establish conventionaltelephone calls via land based networks such as the PSTN. Thus, when thecustomer is in close proximity to their private base station, it ispossible for the customer to obtain the benefits of using a personalmobile telephone while at the same time only experiencing connectioncosts equivalent to those experienced when using a PSTN handset.

[0005] Two significant problems exist with the use of conventional basestation equipment in the role of a private base station. Firstly, theprivate base station must facilitate communication with mobiletelephones in its geographical area while at the same time it isessential that it does not interfere with surrounding cellular networks.Secondly, existing base stations are configured to communicate with aplurality of users therefore although the cost of an individual basestation is relatively high, this cost, over a period of time, is sharedby many users as they enter the geographical area. However, private basestations will tend to provide services for one user or at most arelatively small number of users therefore the overall cost of providingsuch a base station must be substantially reduced if it is to appearcommercially attractive to potential users.

[0006] A conventional base station is disclosed in British patentpublication 2 249 922, wherein communication between a cellular mobiletelephone and a public switch terrestrial network is provided, whileminimising interference with similar base stations. Base stations forboth public and private use are also disclosed in International patentpublication 92/04796.

SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention, there ifprovided a private base station arranged to provide communicationdirectly between a cellular mobile telephone, operable within a macrocellular network, and a public switched terrestrial network; whereinsaid base station includes processing means configurable to transmitsignalling information over an available time slot to establish a call;and said processing means is configurable to transmit user-traffic viasaid available time slot after a call has been established, therebyminimising interference with similar base stations.

[0008] In a preferred embodiment, the processing means is configured tosynchronise said base station to a second base station and thereafteridentify itself as being available to synchronise a third base station.

[0009] According to a second aspect of the present invention, there isafforded a method of providing direct communication between a cellularmobile telephone, operable within a macro cellular network, and a publicswitched terrestrial network, by means of a private base stationincluding processing means, comprising steps executed by said processingmeans of transmitting signalling information over an available time slotto establish a call; and transmitting user traffic via said availabletime slot after a call has been established, thereby minimisinginterference with similar base stations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows a cellular communications network having a pluralityof base stations, a plurality of mobile stations and environments wherecommunication to conventional base stations is impaired;

[0011]FIG. 2 details a mobile telephone communicating with a privatebase station within a cellular microcell;

[0012]FIG. 3 and FIG. 4 illustrates protocols of a first embodiment forcommunicating within the microcell identified in FIG. 2;

[0013]FIG. 5 illustrates procedures for implementing the protocolsidentified in FIGS. 3 and 4;

[0014]FIGS. 6 and 7 illustrate communication protocols in accordancewith a second embodiment;

[0015]FIG. 8 illustrates a protocol for communicating within a microcellin accordance with a third embodiment;

[0016]FIGS. 9 and 10 illustrate synchronising procedures in accordancewith a fourth embodiment;

[0017]FIG. 11 illustrates a frequency hopping procedure in accordancewith a fifth embodiment; and,

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] A digital cellular network is illustrated in FIG. 1, operable inaccordance with the GSM protocol, embodying time division multipleaccess techniques. The network includes a first base station 101 capableof communicating with mobile telephones, such as mobile telephone 102,within a communication region or cell 103. Similarly, mobile telephones,such as 104 and 105 communicate with base station 106 in cell 107, basestation 108 communicates with mobile telephone 109 in cell 110 and cell111 allows mobile telephones 112 and 113 to communicate with basestation 114. Furthermore, as the mobile telephones are moved within thearea of coverage, handovers will occur between base stations. Thus, asmobile telephone 104 is moved away from base station 106 and towardsbase station 101 a handover will occur such that transmissions from themobile telephone to base station 106 will cease and a communication willbe initiated with base station 101, as is well known in the art.

[0019] The cellular network may be considered as providing coveragewithin a built-up area and, given the level of penetration andrequirements for cellular services, it is appreciated that regions existwhere cellular coverage is not provided. Building 115 is positioned insuch a location although a resident of this building regularly travelsinto the built-up area and often makes use of mobile equipment while inthe area covered by the network. However, while resident within building115 it is not possible for the cellular network to be used given that itis not possible for the mobile equipment to establish communicationchannels with any of the nearby base stations, such as base stations 101and 106.

[0020] Mobile telephones 112 and 113 communicate with base station 114when located within cell 111. However, cell 111 includes a number oflarge buildings, such as office block 117 where conventionalcommunication to the base station 114 creates problems. Firstly, due toa high metal content within building 117, radio signals from thebuilding 117 to the base station 114 are severely attenuated thereforein many locations within building 117 it is not possible to establish acommunication channel. Furthermore, it is recognised that building 117contains a relatively high number of mobile users therefore even whenconditions permit connections to be made to base station 114 theavailability of channels rapidly saturates and again customers mayexperience difficulties when communicating via their mobile equipment.

[0021] In order to allow guaranteed use of cellular mobile telephones atbuilding 115 and building 117, residents are provided with their ownprivate base station. The private base stations communicate with mobileequipment but are arranged to provide communication directly to thePSTN, while remaining operable within a cellular macro network. Thus,within the macro coverage region and outside building 117, mobiletelephones, such as telephone 113, operate in a conventional way byestablishing communication to the public base stations, such as basestation 114 which are in turn arranged to establish communicationthrough terrestrial networks etc. However, when a user enters theirprivate domain, communication is improved by the existence of a privatebase station. The private base station is configured such thatcommunication between the private base station and the mobile telephoneadheres to accepted transmission standards. However, the private basestation does not form part of the cellular macro network and establishesa direct communication to terrestrial channels. Thus, under thesecircumstances, the mobile telephone effectively operates as a private“cordless” telephone using the public switch telephone network 116.

[0022] A digital mobile telephone capable of operating in accordancewith the GSM recommendation is shown in FIG. 2, arranged to communicatewith a private base station. The mobile telephone 201 is shown in FIG.2, having a mouth-piece microphone 202, an ear-piece loudspeaker 203,signalling buttons 204 and a liquid crystal display 205. The telephone201 is arranged. to communicate with the base stations via an antenna206 and digital processing of encoded speech signals is effected bymeans of a digital signal processor controlled in response to controlsignals generated by a microcontroller. Alternatively, mobile phonesoperating in accordance with other time division multiple access systemsmay be used such as OCS 1900.

[0023] The mobile telephone is capable of communicating with establishedbase stations, such as base stations 101 etc in the macro cellularnetwork illustrated in FIG. 1. In addition the mobile telephone isconfigured to communicate with a private base station 231, that in turnallows a connection to the conventional PSTN 116 via a base band analogtelephone line 232.

[0024] In the network shown in FIG. 1, it is necessary for each basestation to simultaneously communicate on a plurality of frequencies witha plurality of multiplex digital channels being conveyed on each ofthese frequencies. In normal rate operation eight multiplex channels areprovided on each transmission frequency and in the half rate mode ofoperation it is possible for a total of sixteen channels to betransmitted over a single frequency. However, in the environmentillustrated in FIG. 2, the private base station may only make a singleconnection to the PSTN via analog line 232, therefore at any one time,it is only possible for the base station 231 to communicate with asingle mobile telephone unit, such as unit 201. In accordance withaccepted protocols, an uplink channel 233 is provided from the mobiletelephone 201 to the private base station 231 and a downlink channel234, operating on a different frequency, allows signals to be sent backfrom the private base station 231 to the mobile telephone 201. Thus,this limitation in terms of base station complexity is exploited inorder to substantially reduce the cost of manufacturing such units,thereby making them substantially more attractive to customers,particularly in locations where it is difficult to establish normalcommunications using a cellular network. However, a problem with usingvery simple private base stations of substantially similar design isthat, when placed in relatively close proximity, the base stations havea high probability of suffering from mutual interference.

[0025] Following the established GSM recommendation, the frequenciesemployed for the uplink transmission 233 and the downlink transmission234 do not remain constant, in order to mitigate the effects of radiointerference and attenuation etc. Thus, a burst of information may belost, due to it being transmitted via a frequency having particularlypoor transmission characteristics within the area concerned but the nextburst will be received given that this burst will be transmitted on adifferent frequency. Pseudo random number generators are included whichonce initiated from a particular random seed, result in a specifiedsequence of pseudo random numbers being generated both at a transmitterand at a receiver. Thus, as a transmitter hops to a new transmissionfrequency, a similar hop is effected at the receiver such that thereceiver will be tuned to receive the next transmitted burst. Sequencesof pseudo random numbers are generated for both the uplink transmission233 and the downlink transmission 234, the selected frequencies beingdifferent so as to facilitate the separation of transmission andreception at both stations.

[0026] The GSM protocols allow each transmission frequency to conveyeight time-multiplexed channels thereby allowing each transmissionfrequency to convey eight separate channels of traffic. However, in thearrangement shown in FIG. 2, private base station 231 is only requiredto establish a single communication channel on a first frequency for theuplink 233, with a similar channel being provided, on a differentfrequency, for the downlink 234. Each transmitted channel still occupiesone-eighth of the available capacity in order to maintain similartransmission protocols with the rest of the GSM network but theremaining seven are not actually required for traffic transmission.

[0027] In the wider GSM network a high degree of common channelsignalling is provided to ensure synchronisation of the network and tofacilitate hand-overs as mobile telephones are moved from one cell toanother. In the arrangement shown in FIG. 2 the micronetwork, consistingof the base station 231 and its associated mobile telephone 201, iseffectively isolated from the signalling transmission of the surroundingmacro network, therefore signalling supplied to the mobile telephone 201originates from the private base station 231, while the telephone iswithin the region covered by the micro network.

[0028] An arrangement for facilitating the transmission of signallingdata and user traffic in the environment shown in FIG. 2 is detailed inFIG. 3. A downlink frame 301 is illustrated having eight individual timeslots 302 to 309. The base station 231 only requires one of these timeslots to provide a traffic channel for the downlink 234. The basestation 231 is configured to allocate the second time slot 303 for thispurpose, with the first time slot 302 being used for the transmission ofsignalling information.

[0029] A similar arrangement is provided for the uplink 233 with atransmitted frame 310 having its own eight time slots 311 to 318. Thetransmission of these frames is synchronised such that time slot 302will be transmitted over the uplink in synchronously delayed withrespect to time slot 311 of the downlink 234. A mobile telephone isarranged such that the processing of received data, via the downlink isdisplaced from the processing of transmitted data, via the uplink, suchthat time slots 311 and 312 will not be used for transmission purposes.In order to provide the mobile telephone 202 with a maximum period oftime between transmissions, transmission channels 315 and 316 areselected for the transmission of signalling data and user trafficrespectively. Thus, in both the uplink and downlink transmissionfrequencies, a first time slot has been allocated for signalling and asecond time slot has been allocated for user traffic. Under normalcircumstances, within a macro network, this would be consideredwasteful, given that a one-to-one relationship between traffic andsignalling is not required. However, it is possible for other microcellsto overlap with the first cell, with both cells using similarfrequencies. Such an arrangement is shown in FIG. 4 which represents asecond base station transmitting using frames within the same frequencyband of the frames illustrated in FIG. 3. The second base stationidentifies the transmission frames as being incompletely used and willtherefore capture individual channels for its own transmission purposes.

[0030] As far as the downlink is concerned for the second base station,it is not possible to capture the first two time slots 402 and 403because these time slots are being used by the first base station,illustrated as 302 and 303 in FIG. 3. However, the second base stationis in a position to capture the third and fourth transmission channelsand in doing so it allocates channel 404 for the transmission ofsignalling with channel 405 being allocated for the transmission of usertraffic.

[0031] A similar arrangement is then configured for the uplink with theseventh channel 417 being captured for the transmission of signallinginformation and the eighth channel 418 being captured for thetransmission of user traffic. In the downlink, transmission channels 406to 410 remain unoccupied and may be captured by other base stations withchannels 411 to 414 remaining similarly unoccupied on the uplink andagain being available for capture by other base stations.

[0032] Procedures for implementing a connection within the environmentshown in FIG. 2, in accordance with the procedures identified in FIGS. 3and 4, are detailed in FIG. 5. In the preferred embodiment,communication channels between mobile telephones and private basestations are established when power is initially supplied to the privatebase station. Alternatively, a connection may be initiated by a user,wherein it is necessary for the mobile telephone 201 to initiate a linkto the private base station 231. Alternatively, a conventional telephonecall may signal on line 232 to the private base station 231 which willthen initiate a communication to the mobile telephone 201. Thus, itshould be understood that either the mobile telephone 201 or the basestation 231 may initiate a call and the initiation protocols should beinterpreted in this way.

[0033] At step 501 a recognition is made to the effect that acommunication channel is to be established, resulting in the initiatingstation listening to identify a selected frequency. At step 502 aquestion is asked as to whether a channel is available and if answeredin the negative a further frequency is selected at step 503 and controlis returned to step 501. In remote locations where private base stationsdo not interfere with other private stations, it is unlikely that thequestion asked at step 502 will be answered in the negative.Consequently, control is directed to step 504 where time slots withinthe selected frequency are themselves selected.

[0034] At step 505 a question is asked as to whether two adjacent slotsare available and if answered in the negative control is directed tostep 507 where another frequency is selected and control is returned tostep 501. If two adjacent time slots are available the question asked atstep 505 is answered in the affirmative resulting in control beingdirected to step 508. Signalling information is transmitted at step 508and a communication channel on the second adjacent slot is thenestablished at step 509.

[0035] The procedures illustrated in FIG. 5 are implemented in order tocreate a first communication channel, the uplink 233 when initiated bythe base station, whereafter similar procedures are implemented in orderto establish the associated downlink 234.

[0036] The first preferred embodiment provides a useful protocol forestablishing a microcell using a private base station for isolatedusers, such as a customer located in building 115. However, in the largeoffice building 117 there is a requirement for a relatively large numberof microcells to be created within the building and under thesecircumstances, the protocols identified in the first preferredembodiment would not make best use of the available capacity, given thateach traffic channel will have its own associated signalling channel.

[0037] An alternative arrangement is illustrated in FIG. 6 in which atransmission frequency is used to transmit frames of data, each dividedinto eight time slots 601 to 618. Each microcell will be allocated oneof these channels on respective frequencies to provide communication forits uplink and its downlink. While the telephone and base station,substantially similar to the arrangement shown in FIG. 2, are maintainedin their idle mode, that is to say, the devices are activated but arenot being used for traffic transmission, the allocated time slot, suchas time slot 601 is used for signalling transmission. Thus, time slot601 is used to supply signalling information when used for the uplink toallow the private base station to signal its associated mobile unit.This signalling information will be used to establish a call whereafter,as illustrated by transmitted frame 609, the same time slot 601′ is usedfor the transmission of user generated traffic. This situation continuesuntil the call effectively terminates, whereafter part of thecall-clearing procedure involves re-establishing the channel forsignalling purposes, as illustrated by channel 601″.

[0038] It is possible that other mobile telephones may be allowed toaccess the same private base station therefore when implementing theprocedures identified in FIG. 6 it is not possible for these othermobile stations to establish a call while the signalling channel 601 isbeing used to convey user traffic from another mobile station within themicrocell. Thus, in order to maintain the integrity of the network, itis necessary to issue signalling commands to these other mobilestations, via the signalling channel 601, to the effect that the channelis about to switch over to traffic communication, such that the othermobile stations are instructed to wait for a predetermined time beforeinterrogating a channel to look for further signalling information.

[0039] Procedures for implementing the second preferred embodiment areillustrated in FIG. 7. At step 701 a mobile's respective channel isestablished and signalling data is transmitted effectively informing themobile telephone as to its status. At step 702 a request to initiate acall is made or in response to an incoming call a response isestablished. Thus, the mobile telephone must be activated from its idlestate and signalling information is transmitted over the signallingchannel. At step 703 the signalling channel is used to transmit amessage to all mobiles within the microcell to the effect that they arerequired to wait for a predetermined time before interrogating thecommunication channel to determine whether it has been re-allocated backto signalling purposes. Any other mobile units within the microcell willrespond to this message by waiting for a predetermined interval of timebefore invoking procedures to interrogate signalling information. If onmaking such an interrogation it is determined that signallinginformation continues not. to be available, the mobiles will wait for afurther predetermined period and so on repeatedly until the signallingchannel is established.

[0040] At step 704 a communication channel, such as channel 601, isswitched over to traffic transmission, whereafter traffic communication(usually voice) is performed with a question being asked at step 706 asto whether the call has been cleared. When answered in the negative,control is returned to step 705, with control being restrained withinthe loop of steps 705 and 706 until one of the parties terminates thecall, resulting in the question asked at step 706 being answered in theaffirmative.

[0041] After the question asked at step 706 is answered in theaffirmative, the channel is switched back to a signalling channel atstep 707 and at step 708 a question is asked as to whether the procedureshould continue which, when answered in the affirmative, results incontrol being returned to step 701.

[0042] Thus, by using the same channel for signalling purposes andtraffic communication it is possible to double the transmissioncapacity, which is highly desirable in congested areas such as the areacontained within building 117.

[0043] The procedures identified in the second preferred embodimentincrease the channel capacity such that the channel capacity for atransmission frequency used within a microcell environment becomesequivalent to that in the surrounding cellular network. This is achievedby allowing time slots to communicate in a signalling mode whensignalling information is required then switching over to a traffic modewhen user traffic communication is required. The problem with thisapproach is that it is not possible to supply any signalling informationwhile a call is in progress. In a third preferred embodiment half ratecoding is exploited in which a time slot transmitted during a firstframe is used for signalling whereafter, in the subsequent frame, thesame time slot is used for traffic data. This cycle is then repeated sothat, for a particular time slot, half of the frames transmit signallinginformation with the remaining frames transmitting user traffic. Intheory, the allocation of frames for these two types of transmissioncould be configured with different ratios such that, for example, twoframes could be used for user traffic for each frame of signallinginformation. However, the transmission of user traffic in half rate modeis an established part of the GSM recommendations and coding techniquesare known for facilitating transmission at this rate.

[0044] Half rate coding of the type identified above is illustrated inFIG. 8. 801 represents a sequence of transmitted frames during the idlecondition. Frame 802 is transmitted during a first cycle, followed byframe 803 which is transmitted on the next cycle, followed by frame 804,805 and 806 etc. Frame 802 comprises eight time slot channels 807, 808,809, 810, 811, 812, 813 and 814. In this example time slot 807 has beencaptured for use within a microcell resulting in signalling informationbeing transmitted within this time slot during frame 802. On the nextframe 803 no information is transmitted within time slot 807 but for thenext time slot, time slot 804, signalling information may again betransmitted. Similarly, no information is transmitted during frame 805and signalling information may again be transmitted during frame 806.

[0045] Examples 815 represent the situation after a call has beenconnected allowing user generated traffic to be transmitted in time slot807. Again, during frame 816 signalling information may be transmittedin time slot 807. It is not possible for signalling information to betransmitted during frame 817 and now that a call has been connected,user traffic is transmitted during frame 817. Again, signallinginformation may be transmitted during frame 818, followed by usertraffic being transmitted during frame 819 and signalling informationagain being transmittable during frame 820.

[0046] The arrangement shown in FIG. 8 overcomes the problem of theremoval of all signalling information while still allowing all eighttime slots to be used for the transmission of user traffic. However, inorder to achieve this, it is necessary to reduce the transmissioncapacity for user traffic which will result in a level of signaldegradation.

[0047] The establishment of a call within the environment illustrated inFIG. 6 or the establishment of a call within the environment illustratedin FIG. 8 is substantially similar to the procedures detailed in FIG. 5.However, using these types of transmission, it is only necessary toidentify a single time slot, thereby allowing eight multiplexedtransmissions to occur over the same frequency. Thus, it is necessary tolisten to a selected frequency and to determine whether the frequency isavailable. Thereafter, a single time slot is identified, without theneed to identify two adjacent time slots, whereafter the transmission ofsignalling or traffic is implemented.

[0048] The first, second and third embodiments provide differentsolutions to similar problems associated with providing low cost privatebase stations for use within microcells. The optimum solution willdepend upon the background environment and a private base station couldbe configured to provide all three types of communication with aparticular communication type being selected so as to optimizeperformance for the particular environment in which the private basestation is situated.

[0049] It has been recognized, particularly with reference to the secondpreferred embodiment and the third preferred embodiment, that nearbyprivate base stations may transmit information on the same transmissionfrequency but with different time slot channels being allocated for thetwo micrecells. In the surrounding cellular network this does not createa problem, given that the base stations are mutually synchronised andcontrolled by overseeing management functions. However, inmicro-environments there is no means for providing synchronizationsignals to individual private base stations therefore each base stationis responsible for generating its own timing and synchronizationsignals. In environments where microsystems do not interfere, this doesnot create a problem given that the synchronization of one base stationwill have no effect on surrounding base stations. However, when aplurality of base stations are located in a relatively small region, itis necessary for transmission frequencies to be reused by as manystations as possible by the allocation of transmission channels withinspecified time slots. Under these circumstances, it is necessary for asynchronization scheme to be adopted and without the provision of acentral synchronization source, it is necessary for themicro-environments to mutually synchronize if transmission is takingplace on the same set of frequencies.

[0050] A procedure for effecting mutual synchronization betweeninterfering private base stations is illustrated in FIG. 9. A mobilestation or a private base station needs to identify a transmissionchannel before communication can be effected. The actual communicationmay involve any of the protocols previously described but whencommunicating close to another microsystem, measures must be adopted toensure that the two microsystems do not mutually interfere; this wouldseriously jeopardize the integrity of the communication protocols.

[0051] In FIG. 9, a transmission channel is established when power issupplied to the private base station, by firstly identifying atransmission frequency at step 901. At step 902 a question is asked asto whether channels are in use within the frequency identified at step901. If the question asked at step 902 is answered in the negative, tothe effect that no channels are in use, synchronization is establishedin the first channel and the particular connection is identified asbeing both a master and a slave.

[0052] The particular types of communication or communication states areidentified in FIG. 10. Most communication channels will undergo atransition starting from state 0, when no synchronization has beenestablished because no transmission is taking place. On initiating atransmission path, state 1 is entered because the particularcommunication link under consideration is slaved to another synchronisedchannel. Thereafter, as a new user enters the multiplex, this user willsynchronize to the channel which was itself was the last to join themultiplex, such that the particular channel under consideration which,previously considered as the slave and the last in the chain, now entersstate 2 where it is seen as a master. Eventually, the channel underconsideration will clear and the state will return to base level state0.

[0053] Thus, if no channels are in use, resulting in the question askedat step 902 being answered in the negative, a communication link isestablished in the first channel at step 903; thus the link underconsideration moves from state 0 to state 1 because it has effectivelybeen synchronised for transmission purposes.

[0054] If channels are in use, resulting in the question asked at step902 being answered in the affirmative, control is directed to step 904,resulting in listening procedures being effected in order to analyze thenature of other communication links that have been established withinthe multiplex. When a link is identified a question is asked at step 905as to whether a detected station has been synchronised, thereby placingit into state 1. If this question is answered in the negative, thetelephone is attempting to communicate in frequencies that have not beenset up for this particular type of operation, resulting in control beingdirected to step 910 where another channel is selected. However, if thequestion asked at step 905 is answered in the affirmative, the channelidentified at step 904 has been synchronised to an earlier masterchannel in the chain resulting in a question being asked at step 906 asto whether another channel has been synchronised to the channelidentified at step 904. Thus, question 905 determines whether theidentified channel is itself a slave, so that it may be converted to amaster while step 906 considers whether the identified channel isalready a master. Thus, if the question asked at step 906 is answered inthe affirmative, the identified channel has already been elevated tostate 2 such that it is not an appropriate channel for a further slavechannel to be synchronised thereto. However, if the question asked atstep 906 is answered in the negative, the channel identified at step 904is only in state 1 such that the synchronization may be effected at step907, resulting in the channel identified at step 904 being elevated tostate 2.

[0055] At step 908 a call may be established and at step 909 a questionis asked as to whether the call has been cleared. When answered in thenegative control is returned to step 908 and control is retained withinthe typed loop until a call-clear is effected.

[0056] When the call is cleared, the question asked at step 909 will beanswered in the affirmative, resulting in termination of the process.

[0057] Thus, no one individual private base station acts as a master forsynchronizing stations within its region. A station may establish itselfas a master, whereafter another station may be synchronised to it as aslave. However, as a third communication path is required, this willslave itself to the second communication path, not the original master,such that the title of most eligible master passes around from onestation to the next.

[0058] In accordance with the fourth preferred embodiment, interferencebetween private base stations is minimized by effectively synchronizingthe base stations together such that the timing reference of a firststation will be consistent with the timing reference of nearby stations,thereby allowing the stations to transmit communications innon-colliding time slots, in a manner that emulates the fullysynchronised system of the background cellular network. However, aproblem with the approach of the fourth embodiment is that in somesituations not all of the stations within a particular region may beable to receive transmissions from all of the other base stations withinthat region. Thus, a first station may exist and it may be possible fora second station to receive signals from said first station and therebysynchronize to this first station. Similarly, it may be possible for athird station to synchronize with the first station such that the firststation in combination with the second or the first station incombination with the third would not create any problems. However, it ispossible, perhaps due to the arrangement of dividing walls etc, that thesecond station may not be able to receive transmissions from the thirdand similarly the third may not receive transmissions from the second.Under these circumstances it would be difficult for the role of mosteligible master to be passed from one station to the next, resulting innon-optimum use of available capacity or service failure.

[0059] In a fifth preferred embodiment the problem of synchronizingadjacent micronetworks is completely bypassed and each micronetwork isallowed to establish its own frame synchronization. In order to reduceinterference effects to tolerable levels, all data transmissions,including signalling transmissions, follow a frequency hopping regimewhere each private base station within a particular geographical regionis given a unique frequency hopping sequence, such that any straytransmissions from adjoining regions will be treated as noise and, giventhat this interference will tend to occur for only one particular framewithin many error free frames, the established procedures forinterleaving and error correction should allow the corrupted data to berecovered.

[0060] Procedures for implementing frequency hopping of this type areillustrated in FIG. 11. Each micronetnork will have its own random seed,thereby resulting in a unique sequence of frequency selections beingmade for that particular microcell. Mobile telephones will requiremodification for operating within such an environment and it is possiblethat a user may require a mobile telephone to operate in a plurality ofenvironments of this type, each having their own frequency hoppingsequence. At step 1101 a selection is made identifying the fact that thetelephone will be used within a microcell environment and, if necessary,identifying the particular microcell under consideration. Thus, forexample, a user may programme a mobile telephone with informationidentifying two microcells, possibly one at home with the second in anoffice.

[0061] When operating in this way, the telephone will not become activeimmediately from a power-up condition and it will be necessary for thetelephone to wait until it can identify its position within a repeatedregime of superframes. Each frame includes information identifying thenumber of that frame within the repeated cycle therefore havingsuccessfully received signalling information from one frame, byselecting the correct frequency for that frame, it is possible for themobile station to identify the phase of the frequency hopping sequence.In addition, it is also necessary for the mobile station to synchronizeits own internal clocks to the clock synchronization signals of theprivate base station and in order to achieve this, it is necessary forthe mobile station to receive a frame which includes a synchronizationdata. Thus, at step 1102 a frequency is selected for a particular framenumber which is known to include synchronization data of this type.

[0062] The mobile station remains tuned to the frequency selected atstep 1102 and frame data will be received on the selected frequency.After a frame of data has been received on the frequency selected atstep 1102, a question is asked at step 1103 as to whether the requiredsynchronization data has been transmitted. If this question is answeredin the negative, an incorrect frame has been received, resulting incontrol being directed to step 1104 such that the system waits for thenext frame to be transmitted. If the question asked at step 1103 isanswered in the affirmative, a question is asked at step 1105 as towhether the frame number is correct. Again, if this question is answeredin the negative, control is returned to step 1104 and the system waitsfor the next frame to be transmitted.

[0063] If the question asked at step 1105 is answered in theaffirmative, the mobile station has effectively achieved synchronizationwith its frequency hopping sequence allowing a call to be established atstep 1106. The call progresses at step 1107 and a question is asked atstep 1108 as to whether the call has been cleared. When answered in thenegative, control is returned to step 1107 until the question isanswered in the affirmative, resulting in clearing procedures beinginstigated at step 1109 whereafter the process terminates.

[0064] The procedures illustrated in FIG. 11 allow microcells in closeproximity to co-exist without introducing intolerable level ofinterference. However, in order to operate in this way, it is necessaryto make modification to the cellular telephone such that microcells arenot available to users who have differently configured equipment. Thus,in some environment is may be desirable to adopt the procedures of thefourth preferred embodiment but again this will depend primarily uponunderlying circumstances which may themselves change as new microcellsare introduced into the geographical area.

[0065] In an alternative embodiment the pseudo random sequence istemporarily broken in order to facilitate synchronization between amobile telephone and a private base station. In this way, it is possibleto provide the mobile telephone with more than one burst period duringwhich transmissions occur at the same frequency, ie without a frequencyhop occurring. In this way, more time is provided for the mobiletelephone to synchronize to the base station such that, say after twoburst periods, it is possible for the frequency hopping to be resumedburst-by-burst.

[0066] For example, a frequency hopping sequence may be specified asF1-F2-F3-F4- . . . FN. Preferably, synchronization occurs duringtransmission of frequency F2 therefore the sequence may be broken tofacilitate synchronization at this frequency. Thus, both the mobiletelephone and the private base station would be configured so as tobrake the pseudo random sequence by effectively transmitting data at theF2 frequency for more than one burst period. Thus, the previouslyspecified pseudo random frequency hopping sequence may be replaced bythe following, F1-F2-F2-F4 . . . FN.

1. A private base station arranged to provide communication directlybetween a cellular mobile telephone, operable within a macro cellularnetwork, and a public switched terrestrial network; wherein said basestation includes processing means configurable to transmit signallinginformation over an available time slot to establish a call; and saidprocessing means is configurable to transmit user-traffic via saidavailable time slot after a call has been established, therebyminimising interference with similar base stations.
 2. A private basestation according to claim 1, wherein said processing means isconfigured to synchronise said base station to a second base station andthereafter identify itself as being available to synchronise a thirdbase station.
 3. A method of providing direct communication between acellular mobile telephone, operable within a macro cellular network, anda public switched terrestrial network, by means of a private basestation including processing means, comprising steps executed by saidprocessing means of transmitting signalling information over anavailable time slot to establish a call; and transmitting user-trafficvia said available time slot after a call has been established, therebyminimising interference with similar base stations.
 4. A methodaccording to claim 3, wherein said base station is synchronised to asecond base station and thereafter identifies itself as being availableto synchronise a third base station.